Onium salt, chemically amplified negative resist composition, and pattern forming process

ABSTRACT

An onium salt containing an anion having formula (A1) and a cation having formula (A1-a), (A1-b) or (A1-c) is provided. A chemically amplified negative resist composition comprising the onium salt as acid generator forms a pattern of good profile having a high sensitivity, improved dissolution contrast, reduced LWR and improved CDU.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2020-097357 filed in Japan on Jun. 4, 2020, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to an onium salt, a chemically amplified negative resist composition comprising the same, and a pattern forming process using the resist composition.

BACKGROUND ART

To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film. To form patterns of smaller feature size, research efforts have been made on the resist compositions for lithography using short wavelength radiation, typically electron beam (EB) and extreme ultraviolet (EUV).

With the advance of pattern miniaturization, more attention is paid on the edge roughness (LWR) of line patterns and the critical dimension uniformity (CDU) of hole and dot patterns. The influence of localization and agglomeration of a base polymer or acid generator and the influence of acid diffusion are pointed out. There is a tendency that LWR and CDU are aggravated as resist film becomes thinner. The thickness reduction of resist film to comply with the advance of pattern miniaturization causes degradation of LWR and CDU, which is a serious problem.

The EUV resist composition is required to meet high sensitivity, high resolution and low LWR at the same time. As the acid diffusion distance is reduced, LWR or CDU is improved, but sensitivity becomes lower. For example, as the PEB temperature becomes lower, LWR or CDU is improved, but sensitivity becomes lower. Also, when the amount of quencher added is increased, LWR or CDU is improved, but sensitivity becomes lower. It is necessary to overcome the tradeoff relationship between sensitivity and LWR or CDU (see Patent Document 1).

Resist compositions adapted for photolithography include positive tone compositions wherein exposed regions are dissolved to form patterns and negative tone compositions wherein exposed regions are retained to form patterns. A composition of either tone which is easier to use is selected depending on a particular mode of resist pattern needed. In forming a dot pattern, on use of a positive resist composition adapted for alkaline development, a bright mask must be combined therewith. Undesirably, the mask is readily degraded because a large area must be subjected to exposure to high-energy EB or EUV. The problem is avoided by using a dark mask requiring only a small exposure area. In this case, a negative resist composition of the type wherein exposed regions are insolubilized in alkaline developer is indispensable (see Patent Document 2).

On use of a negative resist composition adapted for alkaline developer, an acid is generated in the exposed region, and the base polymer is insolubilized under the action of the acid. Since the acid generated in the exposed region has high affinity to the alkaline developer, the exposed region is dissolved during development, giving rise to such problems as a pattern top loss and degradations of LWR or CDU. Also, the control of acid diffusion is not regarded sufficient, and the base polymer is insufficiently insolubilized due to diffusion of the acid from the exposed region to the unexposed region. This suggests insufficient resistance to alkaline development, similarly inviting top-loss profiles. The unexposed region to be dissolved in the original sense becomes less alkaline dissolvable, leaving scums.

CITATION LIST

-   Patent Document 1: JP-A 2018-197853 (US 20180335696) -   Patent Document 2: JP-A 2013-164588 (U.S. Pat. No. 9,244,348)

SUMMARY OF INVENTION

There is a need for a negative tone resist composition which falls in the concept of acid-catalyzed chemically amplified resist compositions and which has a high sensitivity and forms a pattern of good profile having reduced values of LWR of line-and-space (LS) patterns or CDU of dot patterns and experiencing minimal top loss.

An object of the invention is to provide a chemically amplified negative resist composition which has a high sensitivity and dissolution contrast and forms a pattern of good profile having reduced LWR and improved CDU, an onium salt serving as an acid generator in the resist composition, and a resist pattern forming process using the resist composition.

The inventors have found that a chemically amplified negative resist composition comprising an acid generator in the form of an onium salt having a specific structure and a specific base polymer has a high sensitivity and a high dissolution contrast due to suppressed dissolution of exposed regions in alkaline developer and forms a pattern of good profile having reduced LWR and improved CDU.

In one aspect, the invention provides an onium salt containing an anion having the formula (A1) and a cation selected from the formulae (A1-a) to (A1-c).

Herein k¹ is an integer of 0 to 2, k² is an integer meeting 0≤k²≤4+4k¹, m¹ is an integer of 1 to 4, m² is each independently an integer of 0 to 4, n is each independently an integer of 0 to 10, p is an integer of 0 to 3, q is an integer of 0 to 2, r is an integer of 0 to 4.

R^(a1) and R^(a2) are each independently hydrogen or a C₁-C₁₀ hydrocarbyl group in which some or all hydrogen atoms may be substituted by halogen and a constituent —CH₂— may be replaced by —O— or —C(═O)—, both R^(a1) and R^(a2) are not hydrogen at the same time, R^(a1) and R^(a2) may bond together to form an aliphatic ring with the carbon atom to which they are attached.

R^(a3) is each independently a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, when n is 2 or more. R²³ may be the same or different and two or more R³ may bond together to form a ring with the atom on W² to which they are attached.

R^(a4) is a C₁-C₁₀ hydrocarbyl group which may contain a heteroatom.

W¹ is a C₃-C₂₀ (n+2)-valent group having a mono- or polycyclic structure, W² is each independently a C₃-C₂₀ (m²+n+1)-valent group having a mono- or polycyclic structure, in which a constituent —CH₂— may be replaced by —O—, —S— or —C(═O)—.

L^(a1) and L^(a2) are each independently a single bond, ether bond, ester bond, sulfonate bond, carbonate bond or carbamate bond.

X^(L) is a single bond or a C₁-C₄₀ hydrocarbylene group which may contain a heteroatom.

R¹, R² and R³ are each independently a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, any two of two R¹ and W² may bond together to form a ring with the sulfur atom in the formula, any two of three R³ and W² may bond together to form a ring with the nitrogen atom in the formula.

In a preferred embodiment, the anion having formula (A1) has the formula (A2):

wherein L^(a1), L^(a2), X^(L), R^(a1) to R^(a4), k¹, k², m¹ and n are as defined above, s is an integer of 0 to 2.

In a preferred embodiment, the anion having formula (A2) has the formula (A3):

wherein L^(a1), X^(L), R^(a1) to R^(a4), k¹, k², m¹, u and s are as defined above.

In another aspect, the invention provides a photoacid generator comprising the onium salt defined above.

In a further aspect, the invention provides a chemically amplified negative resist composition comprising the photoacid generator defined above.

In a preferred embodiment, the chemically amplified negative resist composition further comprises a base polymer comprising recurring units having the formula (B1) and recurring units having the formula (B2).

Herein R^(A) is hydrogen, fluorine, methyl or trifluoromethyl. R¹¹ and R¹² are each independently halogen, an optionally halogenated C₁-C₆ saturated hydrocarbyl group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, or optionally halogenated C₁-C₆ saturated hydrocarbyloxy group. A¹ and A² are each independently a single bond or a C₁-C₁₀ saturated hydrocarbylene group in which a constituent —CH₂— may be replaced by —O—. W¹¹ is hydrogen, a C₁-C₁₀ aliphatic hydrocarbyl group, or an optionally substituted aryl group, a constituent —CH₂— in the aliphatic hydrocarbyl group may be replaced by —O—, —C(═O)—, —O—C(═O)— or —C(═O)—O—. R^(x) and R^(y) are each independently hydrogen, or a C₁-C₁₅ saturated hydrocarbyl group which may be substituted with a hydroxyl or saturated hydrocarbyloxy moiety, or an optionally substituted aryl group, both R^(x) and R^(y) are not hydrogen at the same time, R^(x) and R^(y) may bond together to form a ring with the carbon atom to which they are attached; t¹ and t² are each independently 0 or 1, x¹ and x² are each independently an integer of 0 to 2, a¹ and b¹ are each independently an integer of 1 to 3, a² is an integer meeting 0≤a²≤5+2x¹−a¹, and b² is an integer meeting 0≤b²≤5+2x²−b¹.

In a preferred embodiment, the recurring units of formula (B1) have the formula (B1-1) and the recurring units of formula (B2) have the formula (B2-1):

wherein R^(A), R¹¹, R¹², R^(x), R^(y), W¹¹, a¹ and b¹ are as defined above.

In a preferred embodiment, the base polymer further comprises recurring units of at least one type selected from recurring units having the formula (B3), recurring units having the formula (B4), and recurring units having the formula (B5).

Herein R¹³ and R¹⁴ are each independently hydroxyl, halogen, acetoxy, an optionally halogenated C₁-C₈ alkyl group, optionally halogenated C₁-C₈ saturated hydrocarbyloxy group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyl group, or optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, with the proviso that R³ and R⁴ are not acid labile groups. R¹⁵ is halogen, acetoxy, an optionally halogenated C₁-C₈ alkyl group, optionally halogenated C₁-C₈ saturated hydrocarbyloxy group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyl group, or optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, with the proviso that R¹⁵ is not an acid labile group; c, d and e are each independently an integer of 0 to 4.

In a preferred embodiment, the base polymer further comprises recurring units of at least one type selected from recurring units having the formula (B6), recurring units having the formula (B7), and recurring units having the formula (B8).

Herein R^(B) is each independently hydrogen or methyl. Z¹ is a single bond, or a C₁-C₆ is aliphatic hydrocarbylene group, phenylene, naphthylene or a C₇-C₁₈ group obtained by combining the foregoing, or —O—Z¹¹—, —C(═O)—O—Z¹¹— or —C(═O)—N—H—Z¹¹—, Z¹¹ is a C₁-C₆ aliphatic hydrocarbylene group, phenylene, naphthylene or a C₇-C₁₈ group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety. Z² is a single bond or —Z²¹—C(═O)—O—, wherein Z²¹ is a C₁-C₂₀ hydrocarbylene group which may contain a heteroatom. Z³ is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene, —O—Z³¹—, —C(═O)—O—Z³¹— or —C(═O)—NH—Z³¹—, wherein Z³¹ is a C₁-C₁₀ aliphatic hydrocarbylene group, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene, or a C₇-C₂₀ group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety. R²¹ to R²⁸ are each independently halogen or a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, a pair of R²¹ and R²², R²³ and R²⁴, or R²⁶ and R²⁷ may bond together to form a ring with the sulfur atom to which they are attached. M⁻ is a non-nucleophilic counter ion.

In a preferred embodiment, the base polymer comprises recurring units having the formula (B1-2), recurring units having the formula (B2-2), and recurring units having the formula (B7):

wherein R^(A), R^(B), Z², R²³ to R²⁵, R^(x), R^(y), W¹¹, a¹ and b¹ are as defined above.

In a preferred embodiment, the base polymer comprises recurring units having the formula (B1) and recurring units having the formula (B2), but not recurring units having the formula (B6), recurring units having the formula (B7) and recurring units having the formula (B8).

The chemically amplified negative resist composition may further comprise an organic solvent, a quencher, and/or a crosslinker.

In another preferred embodiment, the resist composition is free of a crosslinker.

The chemically amplified negative resist composition may further comprise a fluorinated polymer comprising recurring units having the formula (D1) and recurring units of at least one type selected from recurring units having the formulae (D2) to (D5).

Herein R^(C) is each independently hydrogen or methyl; R^(D) is each independently hydrogen, fluorine, methyl or trifluoromethyl; R³⁰¹ is hydrogen or a C₁-C₅ straight or branched hydrocarbyl group which may have a heteroatom-containing moiety intervening in a carbon-carbon bond; R³⁰² is a C₁-C₅ straight or branched hydrocarbyl group which may have a heteroatom-containing moiety intervening in a carbon-carbon bond; R³⁰³, R³⁰⁴, R³⁰⁶ and R³⁰⁷ are each independently hydrogen or a C₁-C₁₀ saturated hydrocarbyl group; R³⁰⁵, R³⁰⁸, R³⁰⁹ and R³¹⁰ are each independently hydrogen, a C₁-C₁₅ hydrocarbyl group, C₁-C₁₅ fluorinated hydrocarbyl group, or acid labile group, an ether bond or carbonyl moiety may intervene in a carbon-carbon bond in the hydrocarbyl or fluorinated hydrocarbyl group; g¹ is an integer of 1 to 3, g² is an integer meeting 0≤g²≤5+2g³−g¹, g³ is 0 or 1, h is an integer of 1 to 3; X¹ is a single bond, —C(═O)—O— or —C(═O)—NH—; and X² is a C₁-C₂₀ (h+1)-valent hydrocarbon group or C₁-C₂₀ (h+1)-valent fluorinated hydrocarbon group.

In a still further aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the chemically amplified negative resist composition defined above onto a substrate to form a resist film thereon, exposing patternwise the resist film to high-energy radiation, and developing the exposed resist film in an alkaline developer.

Typically, the high-energy radiation is EB or EUV of wavelength 3 to 15 nm.

Advantageous Effect of Invention

The onium salt of the invention is characterized by having a tertiary or secondary hydroxyl group in the cation and/or anion structure and functions as a photoacid generator. Although the unexposed region remains highly hydrophilic because of the hydroxyl-containing structure, the exposed region turns to a hydrophobic structure through the mechanism that once an acid is generated upon exposure, the tertiary or secondary hydroxyl group in the cation and/or anion structure undergoes dehydration reaction. Then the dissolution contrast between exposed and unexposed regions is improved. The base polymer has a high dissolution contrast and a high insolubilizing ability in the exposed region. By virtue of these advantages, a chemically amplified negative resist composition having a high sensitivity and improved LWR or CDU can be designed. Since a polarity switch takes place not only on the base polymer, but also on the photoacid generator, the dissolution of the composition in alkaline developer is reduced. The negative pattern formed from the inventive chemically amplified negative resist composition is restrained in the dissolution of exposed region in alkaline developer and forms a pattern of good profile with minimal top loss, as compared with negative patterns formed from conventional negative resist compositions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the ¹H-NMR spectrum of PAG-1 prepared in Example 1-1.

FIG. 2 is a graph showing a percent film retention as a function of exposure dose.

DESCRIPTION OF PREFERRED EMBODIMENTS

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. “Optional” or “optionally” means that the subsequently described event or circumstances may or may not occur, and that description includes instances where the event or circumstance occurs and instances where it does not. The notation (Cn-Cm) means a group containing from n to in carbon atoms per group. In chemical formulae, Me stands for methyl, Ac for acetyl, and the broken line designates a valence bond.

The abbreviations and acronyms have the following meaning.

EB: electron beam

EUV: extreme ultraviolet

Mw: weight average molecular weight

Mn: number average molecular weight

Mw/Mn: molecular weight dispersity

GPC: gel permeation chromatography

PEB: post-exposure bake

PAG: photoacid generator

LWR: line width roughness

LER: line edge roughness

CDU: critical dimension uniformity

EL: exposure latitude

Onium Salt

One embodiment of the invention is an onium salt containing an anion having the following formula (A1) and a cation having the following formula (A1-a), (A1-b) or (A1-c).

In formulae (A1) and (A1-a) to (A1-c), k¹ is an integer of 0 to 2, preferably 0 or 1, more preferably 0. The subscript k² is an integer meeting 0≤k²≤4+4k¹. From the aspect of introducing a substituent group into the onium salt to control the diffusibility of an acid generated therefrom upon exposure, k² is preferably an integer of 0 to 4, more preferably 2 or 3. The subscript m¹ is an integer of 1 to 4, m² is each independently an integer of 0 to 4, n is each independently an integer of 0 to 10, p is an integer of 0 to 3, q is an integer of 0 to 2, and r is an integer of 0 to 4,

R^(a1) and R^(a2) are each independently hydrogen or a C₁-C₁₀ hydrocarbyl group in which some or all of the hydrogen atoms may be substituted by halogen atoms such as fluorine, chlorine, bromine or iodine and a constituent —CH₂— may be replaced by —O— or —C(═O)—. Both R^(a1) and R^(a2) are not hydrogen at the same time. R^(a1) and R^(a2) may bond together to form an aliphatic ring with the carbon atom to which they are attached.

The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, text-pentyl, n-hexyl, 2-ethylhexyl, n-octyl, n-nonyl and n-decyl; cyclic saturated hydrocarbyl groups such as cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexyhnethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl and adamantyl; alkenyl groups such as vinyl; and aryl groups such as phenyl, tolyl and naphthyl. Examples of the substituted hydrocarbyl group include oxanorbornyl and fluorophenyl. R^(a1) and R^(a2) are preferably identical, most preferably both methyl.

Examples of the aliphatic ring that R^(a1) and R^(a2), taken together, form with the carbon atom to which they are attached include cyclopropane, cyclobutane, cyclopentane, and cyclohexane rings, but are not limited thereto.

In formulae (A1) and (A1-a) to (A1-c), R²³ is each independently a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom. The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, tert-pentyl, n-hexyl, n-octyl, 2-ethylhexyl, n-nonyl, and n-decyl; cyclic saturated hydrocarbyl groups such as cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, adamantyl, and adamantylmethyl; and aryl groups such as phenyl, naphthyl and anthracenyl. In these hydrocarbyl groups, some or all of the hydrogen atoms may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, or a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene in a carbon-carbon bond, so that the group may contain a hydroxyl moiety, cyano moiety, carbonyl moiety, ether bond, ester bond, sulfonic acid ester bond, carbonate bond, carbamate bond, amide bond, imide bond, lactone ring, sultone ring, thiolactone ring, lactam ring, sultam ring, carboxylic anhydride, or haloalkyl moiety.

When n is 2 or more, groups R^(a3) may be the same or different and two or more groups R^(a3) may bond together to form a ring with the atom on W² to which they are attached. Examples of the ring thus formed include cyclopropane, cyclobutane, cyclopentane, cyclohexane, norbornane, and adamantane rings. It is acceptable that two groups R^(a3) bond with a common carbon atom and bond together to form a spiro ring structure with W¹ or W².

In formula (A1), R^(a4) is a C₁-C₁₀ hydrocarbyl group which may contain a heteroatom. The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include alkyl groups such as methyl, ethyl propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, tert-pentyl, n-hexyl, n-octyl, 2-ethylhexyl, n-nonyl, and n-decyl: cyclic saturated hydrocarbyl groups such as cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexyhnethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, and adamantyl: alkenyl groups such as vinyl: and aryl groups such as phenyl, tolyl and naphthyl. In these hydrocarbyl groups, some or all of the hydrogen atoms may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, or a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene in a carbon-carbon bond, so that the group may contain a hydroxyl moiety, cyano moiety, carbonyl moiety, ether bond, ester bond, sulfonic acid ester bond, carbonate bond, carbamate bond, amide bond, imide bond, lactone ring, sultone ring, thiolactone ring, lactam ring, sultam ring, carboxylic anhydride, or haloalkyl moiety. Suitable heteroatom-containing hydrocarbyl groups include oxanorbornyl and fluorophenyl.

In formula (A1), R^(a4) is preferably situated at the ortho-position relative to the SO₃ ⁻ group because the SO₃ ⁻ group, that is, the site acting as an acid is then blocked by steric bulkiness so that an effect of apparently suppressing acid diffusion is exerted.

In formulae (A1) and (A1-a) to (A1-c), W¹ is a C₃-C₂₀ (n+2)-valent group having a mono- or polycyclic structure, W² is each independently a C₃-C₂₀ (m+n+1)-valent group having a mono- or polycyclic structure. In these groups, a constituent —CH₂— may be replaced by —O—, —S— or —C(═O)—. Examples of the group having a mono- or polycyclic structure are shown below, but not limited thereto.

Of these, the groups having an aromatic ring are preferred for availability of starting reactants, with the groups having a benzene or naphthalene ring being more preferred.

In formula (A1), L^(a1) and L^(a2) are each independently a single bond, ether bond, ester bond, sulfonate bond, carbonate bond or carbamate bond. Of these, an ether or ester bond is preferred, with the ester bond being most preferred.

In formula (A1), X^(L) is a single bond or a C₁-C₄₀ hydrocarbylene group which may contain a heteroatom. Examples of the optionally heteroatom-containing C₁-C₄₀ hydrocarbylene group are shown below, but not limited thereto. Herein, * designates a point of attachment to L^(a1) or L^(a2).

Of these, the groups X^(L)-0 to X^(L)-22 and X^(L)-47 to X^(L)-49 are preferred, with the groups X^(L)-0 to X^(L)-17 being mon preferred.

Of the anions having formula (A1), those having the formula (A2) are preferred.

Herein L^(a1), L^(a2), X^(L), R^(a1) to R^(a4), k¹, k², m¹ and n are as defined above, s is an integer of 0 to 2.

Of the anions having formula (A2), those having the formula (A3) are preferred.

Herein L^(a1), X^(L), R^(a1) to R^(a4), k¹, k², m¹, n and s are as defined above.

Examples of the anion having formula (A1) are shown below, but not limited thereto. Herein R^(a4) and k² are as defined above.

The cation of formula (A1-a) is a sulfonium cation. In formula (A1-a), R¹ is a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom. The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl; cyclic saturated hydrocarbyl groups such as cyclopropyl, cyclopentyl, cyclohexyl, cyclopropylmethyl, 4-methylcyclohexyl, cyclohexylmethyl, norbornyl and adamantyl; alkenyl groups such as vinyl, allyl, propenyl, butenyl and hexenyl; cyclic unsaturated aliphatic hydrocarbyl groups such as cyclohexenyl; aryl groups such as phenyl and naphthyl; heteroaryl groups such as thienyl; and aralkyl groups such as benzyl, 1-phenylethyl and 2-phenylethyl. Of these, aryl groups are preferred. In the foregoing hydrocarbyl groups, some or all of the hydrogen atom may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, or a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene in a carbon-carbon bond, so that the group may contain a hydroxyl moiety, cyano moiety, carbonyl moiety, ether bond, ester bond, sulfonic acid ester bond, carbonate bond, lactone ring, sultone ring, carboxylic anhydride or haloalkyl moiety.

Any two of two R¹ and W² may bond together to form a ring with the sulfur atom in the formula. Examples of the ring structure are shown below.

Examples of the sulfonium cation having formula (A1-a) are shown below, but not limited thereto.

The cation of formula (A1-b) is an iodonium cation. In formula (A1-b), R² is a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom. The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof are as exemplified above for R¹.

Examples of the iodonium cation having formula (A1-b) are shown below, but not limited thereto.

The cation of formula (A1-c) is an ammonium cation. In formula (A1-c), R³ is a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom. The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof are as exemplified above for R¹.

Examples of the ammonium cation having formula (A1-c) we shown below, but not limited thereto.

The onium salt of the invention may be prepared, for example, according to the following scheme. Although reference is made to the synthesis of an onium salt having the following formula (A1-1) as a typical example, the synthesis method is not limited thereto.

Herein, W¹, W², R^(a1) to R^(a4), R¹, k¹, k², m¹, m², n and p are as defined above. R⁰⁰ is a C₁-C₃ hydrocarbyl group, X^(Ha1) is a halogen atom other than fluorine, M⁺ is a counter cation, and Z⁻ is a counter anion.

The first step is to react an Intermediate In-A, which is commercially available or may be synthesized by any well-known organic synthesis method, with a Grignard reagent or organolithium reagent to form an Intermediate In-B. The reaction may be conducted by any well-known organic synthesis method. Specifically, the Grignard reagent or organolithium reagent is diluted with a solvent such as tetrahydrofuran (THF) or diethyl ether, to which a solution of Intermediate In-A in a solvent such as THF or diethyl ether is added dropwise. The amount of the Grignard reagent or organolithium reagent used is preferably (2 m¹+2) equivalents, with the number of ester bonds in Intermediate In-A and the loss by deactivation with hydroxyl group being taken into account. The reaction temperature is preferably in the range from room temperature to approximately the boiling point of the solvent. The reaction time is determined as appropriate by monitoring the reaction process by gas chromatography (GC) or silica gel thin-layer chromatography (TLC) because it is desirable from the yield aspect to drive the reaction to completion. The reaction time is typically about 30 minutes to about 2 hours. Intermediate In-B may be recovered from the reaction mixture by ordinary aqueous work-up. If necessary, it may be purified by standard techniques like chromatography and recrystallization.

The second step is to react Intermediate In-B with a haloacetate compound to form an Intermediate In-C. The reaction may be conducted by any well-known organic synthesis method. Specifically, Intermediate In-B and a base are dissolved in a polar aprotic solvent such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), to which the haloacetate compound is added dropwise. Examples of the base used herein include inorganic bases such as potassium carbonate and cesium carbonate and organic bases such as triethylamine and N,N-diisopropylethylamine. On use of the haloacetate compound in which the halogen atom is chlorine or bromine, the reaction rate may be accelerated by adding a catalytic amount of sodium iodide or potassium iodide. The reaction temperature is preferably in the range from room temperature to 100° C. The reaction time is determined as appropriate by monitoring the reaction process by GC or TLC because it is desirable from the yield aspect to drive the reaction to completion. The reaction time is typically about 2 to about 12 hours. Intermediate In-C may be recovered from the reaction mixture by ordinary aqueous work-up. If necessary, it may be purified by standard techniques like chromatography and recrystallization.

The 3rd step is hydrolysis of Intermediate In-C to form an Intermediate In-D. The reaction may be conducted by any well-known method. Specifically, Intermediate In-C is dissolved in 1,4-dioxane or the like, to which a base is added dropwise. The base used herein is typically selected from aqueous solutions of inorganic bases such as sodium hydroxide and potassium hydroxide. The reaction temperature is preferably in the range from room temperature to 60° C. The reaction time is determined as appropriate by monitoring the reaction process by TLC because it is desirable from the yield aspect to drive the reaction to completion. The reaction time is typically about 2 to about 12 hours. At the end, the reaction is quenched by adding an acid such as an aqueous solution of hydrochloric acid or nitric acid. Intermediate In-D may be recovered from the reaction mixture by ordinary aqueous work-up. If necessary, it may be purified by standard techniques like chromatography and recrystallization.

The 4th step is to react Intermediate In-D with an Intermediate In-E to form an Intermediate In-F. In the reaction of the carboxyl group of Intermediate In-D with the hydroxyl group of Intermediate In-E to form an ester bond directly, a condensing agent may be used. Examples of the condensing agent used herein include N,N′-dicyclohexylcarbodiimide, N,N′-diisopropylcarbodiimide, 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide, and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. Of these, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride is preferably used because the urea compound which is formed as a by-product at the end of reaction is easily removed. The reaction is typically carried out by dissolving Intermediate In-D and Intermediate In-E in a halogen base solvent such as methylene chloride and adding a condensing agent to the solution. The reaction rate may be accelerated by adding N,N-dimethyl-4-dimethylaminopyridine as a catalyst. The reaction time is determined as appropriate by monitoring the reaction process by TLC because it is desirable from the yield aspect to drive the reaction to completion. The reaction time is typically about 12 to about 24 hours. At the end of reaction, the urea compound formed as by-product is removed by filtration or water washing, if necessary. Intermediate In-F may be recovered from the reaction solution by ordinary aqueous work-up. If necessary, it may be purified by standard techniques like chromatography and recrystallization.

The 5th step is an ion exchange of Intermediate In-F with an Intermediate In-G to yield an onium salt (A1-1). Intermediate In-G wherein Z⁻ is chloride ion, bromide ion, iodide ion or methylsulfate anion is preferred for quantitative progress of the exchange reaction. In view of yield, the progress of reaction is preferably monitored by T. Onium salt (A1-1) may be recovered from the reaction mixture by ordinary aqueous work-up. If necessary, it may be purified by standard techniques like chromatography and recrystallization. The ion exchange of the 5th step may be readily conducted by any well-known method with reference to JP-A 2007-145797, for example.

The preparation method according to the aforementioned scheme is merely exemplary, and the method for preparing the inventive onium salt is not limited thereto. While the aforementioned scheme refers to the synthesis of an ester bond-containing compound, an onium salt having an ether bond, sulfonate bond, carbonate bond or carbamate bond can be synthesized by the skilled artisan using an organic chemistry method within the range of common knowledge.

Resist Composition

Another embodiment of the invention is a chemically amplified negative resist composition which essentially contains a photoacid generator in the form of the onium salt defined above. Preferably the resist composition contains a base polymer of specific structure as well as the onium salt.

In the resist composition, the photoacid generator in the form of the onium salt is present in an amount of preferably 0.01 to 100 parts by weight, more preferably 0.05 to 50 parts by weight per 100 parts by weight of the base polymer, as viewed from the standpoints of sensitivity and acid diffusion controlling effect.

Base Polymer

The base polymer in the resist composition is a polymer comprising recurring units having the following formula (B1) and recurring units having the following formula (B2), which are also referred to as recurring units (B1) and (B2), respectively.

The recurring units (B1) are units for establishing etch resistance and for providing adhesion to substrates and dissolution in alkaline developer. The recurring units (B2) are units for allowing the acid labile group to undergo elimination reaction under the action of acid which is generated from the acid generator upon exposure to high-energy radiation, for thereby inducing alkaline insolubilization and crosslinking reaction among polymer molecules. The recurring units (B2) act to promote the progress of negative reaction to enhance resolution performance.

In formulae (B) and (B2), R^(A) is hydrogen, fluorine, methyl or trifluoromethyl.

R¹¹ and R¹² are each independently halogen, an optionally halogenated C₁-C₆ saturated hydrocarbyl group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, or optionally halogenated C₁-C₆ saturated hydrocarbyloxy group. The saturated hydrocarbyl group and the saturated hydrocarbyl moiety in the saturated hydrocarbylcarbonyloxy group and saturated hydrocarbyloxy group may be straight, branched or cyclic. Examples thereof include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, pentyl and hexyl, cycloalkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl, and a combination thereof. As long as the carbon count does not exceed the upper limit, the polymer is fully dissolvable in alkaline developer.

In formulae (B1) and (B2), A¹ and A² are each independently a single bond or a C₁-C₁₀ saturated hydrocarbylene group in which a constituent —CH₂— may be replaced by —O—. The saturated hydrocarbylene group may be straight, branched or cyclic and examples thereof include alkanediyl groups such as methylene, ethane-1,1-diyl, ethane-1,2-diyl, propane-1,2-diyl, propane-1,3-diyl, butane-1,4-diyl, pentane-1,5-diyl, and hexane-1,6-diyl; cyclic saturated hydrocarbylene groups such as cyclopropanediyl, cyclobutanediyl, cyclopentanediyl, and cyclohexanediyl: and combinations thereof. For the saturated hydrocarbylene group containing an ether bond, in case of t¹=1 in formula (B1), the ether bond may be situated at any position excluding the position between the carbon at α-position and the carbon at β-position relative to the ester oxygen. In case of t¹=0, the atom attached to the main chain becomes the ether oxygen atom, and a second ether bond may be situated at any position excluding the position between the carbon at α-position and the carbon at pi-position relative to the ether oxygen atom. Since the carbon count of the saturated hydrocarbylene group is 10 or less, the polymer is fully dissolvable in alkaline developer.

In formula (B2), W¹¹ is hydrogen, a C₁-C₁₀ aliphatic hydrocarbyl group, or an optionally substituted aryl group. The aliphatic hydrocarbyl group may be straight, branched or cyclic and examples thereof include alkyl groups such as methyl, ethyl, propyl, isopropyl, and cyclic aliphatic hydrocarbyl groups such as cyclopentyl, cyclohexyl and adamantyl. Typical of the aryl group is phenyl. A constituent —CH₂— in the aliphatic hydrocarbyl group may be replaced by —O—, —C(═O)—, —O—C(═O)— or —C(═O)—O—. The constituent —CH₂— in the hydrocarbyl group may bond to the oxygen atom in formula (B2). Typical of the group after such replacement is methylcarbonyl.

In formula (B2), R¹ and R⁷ are each independently hydrogen, or a C₁-C₁₅ saturated hydrocarbyl group which may be substituted with a hydroxyl or saturated hydrocarbyloxy moiety, or an optionally substituted aryl group. Both R^(x) and R^(y) are not hydrogen at the same tune. R^(x) and R^(y) may bond together to form a ring with the carbon atom to which they are attached. The saturated hydrocarbyl group may be straight, branched or cyclic and examples thereof include alkyl groups such as methyl, ethyl, propyl, butyl and structural isomers thereof, and substituted forms of these groups in which some hydrogen is substituted by a hydroxyl moiety or saturated hydrocarbyloxy moiety.

In formulae (B1) and (B2), t¹ and t² are each independently 0 or 1. The subscripts x¹ and x² are each independently an integer of 0 to 2, and the relevant structure represents a benzene skeleton in case of x¹ or x²=0, a naphthalene skeleton in case of x¹ or x²=1, and an anthracene skeleton in case of x¹ or x²=2. The subscript “a²” is an integer meeting 0≤a²≤5+2x¹−a¹, b² is an integer meeting 0≤b²≤5+2x²−b¹, a¹ and b¹ are each independently an integer of 1 to 3. In case of x¹=0, preferably “a²” is an integer of 0 to 3, and a¹ is an integer of 1 to 3; in case of x¹=1 or 2, preferably “a²” is an integer of 0 to 4, and a¹ is an integer of 1 to 3. In case of x²=0, preferably b² is an integer of 0 to 3, and b¹ is an integer of 1 to 3; in case of x²=1 or 2, preferably b² is an integer of 0 to 4, and b¹ is an integer of 1 to 3.

In the embodiment wherein t¹=0 and A1 is a single bond, that is, the aromatic ring directly bonds to the polymer main chain, or differently stated, the unit is free of the linker (—CO—O-A¹-), the recurring units (B1) are preferably recurring units having the following formula (B1-1), which are also referred to as recurring units (B1-1), hereinafter.

Herein R^(A), R¹¹ and a¹ are as defined above.

Preferred examples of the recurring unit (B1) include units derived from 3-hydroxystyrene, 4-hydroxystyrene, 5-hydroxy-2-vinylnaphthalene, and 6-hydroxy-2-vinylnaphthalene. Units of the formulae shown below are more preferred.

Herein R^(A) is as defined above.

In the embodiment wherein t¹=1, that is, the unit has an ester bond as the linker, preferred examples of the recurring unit (B1) are shown below, but not limited thereto.

Herein R^(A) is as defined above.

The recurring units (B2) are preferably recurring units having the following formula (B2-1), which are also referred to as recurring units (B2-1), hereinafter.

Herein R^(A), R¹², R^(x), R^(y), W¹ and b¹ are as defined above.

Preferred examples of the recurring unit (B2) we shown below, but not limited thereto.

The content of recurring units is determined so as to establish a high contrast between the exposed region (which turns negative) and the unexposed region (which does not turn negative) upon exposure to high-energy radiation, for the purpose of obtaining a high resolution. As to the content of recurring units (B1), the lower limit is preferably 30 mol %, more preferably 40 mol %, even more preferably 50 mol %, and the upper limit is preferably 95 mol %, more preferably 90 mol %, even more preferably 80 mol %, based on the overall recurring units of the base polymer.

As to the content of recurring units (B2), the lower limit is preferably 5 mol %, more preferably 10 mol %, even more preferably 20 mol %, and the upper limit is preferably 70 mol %, more preferably 60 mol %, even more preferably 50 mol %, based on the overall recurring units of the base polymer.

Particularly when the base polymer contains recurring units (B1-1) and (B2-1), the recurring units (B1-1) contribute to a further improvement in etch resistance and improvements in adhesion to the substrate and dissolution in alkaline developer whereas the recurring units (B2-1) contribute to a more efficient progress of negative reaction and a further improvement in resolution.

For the purpose of enhancing etch resistance, the base polymer may further comprise recurring units of at least one type selected from recurring units having the formula (B3), recurring units having the formula (B4), and recurring units having the formula (B5). It is noted that these units are also referred to as recurring units (B3), (B4) and (B5), hereinafter.

In formulae (B3) and (B4), R¹³ and R¹⁴ are each independently hydroxyl, halogen, acetoxy, an optionally halogenated C₁-C₈ alkyl group, optionally halogenated C₁-C₈ saturated hydrocarbyloxy group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyl group, or optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, with the proviso that R¹³ and R¹⁴ are not acid labile groups.

In formula (B5), R¹⁵ is halogen, acetoxy, an optionally halogenated C₁-C₈ alkyl group, optionally halogenated C₁-C₈ saturated hydrocarbyloxy group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyl group, or optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, with the proviso that R¹⁵ is not an acid labile group.

In formulae (B3) to (B5), c, d and e are each independently an integer of 0 to 4.

When constituent units of at least one type selected from recurring units (B3) to (B5) are included, not only the aromatic ring inherently possesses etch resistance, but the ring structure incorporated in the main chain also exerts the effect of improving etch resistance and resistance to EB which is irradiated for pattern inspection.

Of the recurring units (B3) to (5), units of one type or a mixture of two or more types may be used. The content of recurring units (B3) to (B5) is determined so as to achieve the effect of improving etch resistance, and its lower limit is preferably 2 mol %, more preferably 5 mol % and its upper limit is preferably 30 mol %, more preferably 20 mol %, based on the overall recurring units of the base polymer.

The base polymer may further comprise recurring units of at least one type selected from recurring units having the formula (B6), recurring units having the formula (B7), and recurring units having the formula (B8). It is noted that these units are also referred to as recurring units (B6), (B7) and (B8), hereinafter.

In formulae (B6) to (B8), R^(B) is each independently hydrogen or methyl. Z¹ is a single bond, or a C₁-C₆ aliphatic hydrocarbylene group, phenylene, naphthylene or a C₇-C₁₈ group obtained by combining the foregoing, or —O—Z¹¹—, —C(═O)—O—Z¹¹— or —C(═O)—NH—Z¹¹—, wherein Z¹¹ is a C₁-C₆ aliphatic hydrocarbylene group, phenylene, naphthylene or a C₇-C₁₈ group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety. Z² is a single bond or —Z²¹—C(═O)—O—, wherein Z²¹ is a C₁-C₂₀ hydrocarbylene group which may contain a heteroatom. Z³ is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene, —O—Z³¹—, —C(═O)—O—Z³¹— or —C(═O)—NH—Z³¹—, wherein Z³¹ is a C₁-C₁₀ aliphatic hydrocarbylene group, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene, or a C₇-C₂₀ group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety.

In formula (B7) wherein Z² is —Z²¹—C(═O)—O—, examples of the optionally heteroatom-containing C₁-C₂₀ hydrocarbylene group represented by Z²¹ are shown below, but not limited thereto.

In formulae (B6) to (B8), R²¹ to R²⁸ are each independently halogen or a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, a pair of R²¹ and R²², R²³ and R²⁴, or R²⁶ and R⁷² may bond together to form a ring with the sulfur atom to which they are attached. Examples of the optionally heteroatom-containing hydrocarbyl group are as exemplified above for R¹.

When a pair of R²¹ and R²², R²³ and R²⁴, or R²⁶ and R²⁷, taken together, form a ring with the sulfur atom to which they are attached, examples of the ring are shown below, but not limited thereto.

Illustrative structures of the sulfonium cation in formulae (B7) and (B8) are shown below, but not limited thereto.

In formula (B6), M⁻ is a non-nucleophilic counter ion.

The recurring units (B6) to (B) are capable of generating an acid upon exposure to high-energy radiation. The incorporation of these units in the polymer provides for appropriate suppression of acid diffusion, thus forming a pattern with reduced LER. The incorporation of these units is also effective for suppressing the phenomenon that the acid volatilizes from the exposed region and deposits on the unexposed region again during bake in vacuum, which is effective for reducing LER and suppressing unwanted negative reaction in the unexposed region for thereby minimizing defects. When the base polymer contains recurring units (B6) to (B), their content is preferably 0.5 to 20 mol % of the overall recurring units.

The base polymer may contain (meth)acrylate units which are protected with conventional acid labile groups, and (meth)acrylate units having an adhesive group such as lactone structure. The incorporation of these recurring units enables fine adjustment of various properties of a resist film, though they are not essential.

Of these, the (meth)acrylate units having an adhesive group include, for example, units having the following formulae (B9) to (B11). These units may be used in a complimentary manner as units (exhibiting no acidity) for imparting adhesion to the substrate or units for adjusting solubility.

Herein R^(A) is as defined above, R³¹ is —O— or —CH₂—, R³² is hydrogen or hydroxyl, R³³ is a C₁-C₄ saturated hydrocarbyl group, and f is an integer of 0 to 3.

The content of these recurring units is preferably 0 to 30 mol %, more preferably 0 to 20 mol % of the overall recurring units.

The preferred base polymer is a polymer comprising recurring units having the formula (B1-2), recurring units having the formula (B2-2), and recurring units having the formula (137).

Herein R^(A), R^(B), Z², R²³ to R²⁵, R^(x), R^(y), W¹, a¹ and b¹ are as defined above.

A mixture of a first polymer free of recurring units (B6) to (B8) and a second polymer comprising recurring units (B6), (B7) or (B8) may also be used as the base polymer. In this embodiment, the amount of the first polymer free of recurring units (136) to (138) is preferably 2 to 5,000 parts by weight, more preferably 10 to 1,000 parts by weight per 100 parts by weight of the second polymer comprising recurring units (B6), (B7) or (B8). It is noted that the first and second polymers contain recurring units (B1) and (B2).

In one embodiment of the invention wherein the chemically amplified negative resist composition is used in the manufacture of photomasks, the composition is coated to a thickness of up to 150 nm, preferably up to 100 n in the advanced generation of lithography. Since a strong development process is often taken, in the processing of the resist composition comprising the base polymer, for minimizing defects resulting from resist residues, the rate of dissolution of the polymer in an alkaline developer, typically 2.38 wt % tetramethylammonium hydroxide (TMAH) aqueous solution is preferably adjusted to 80 nm/sec or less, more preferably 50 nm/sec or less, in order to form a pattern with a small future size. In another example wherein an LSI chip is fabricated from a wafer, the chemically amplified negative resist composition is used in the EUV exposure process, and the coating thickness is often set to 100 nm or less because fine lines of 50 nm or less must be patterned. When the resist film is so thin, the pattern can be degraded by development. Then the rate of dissolution of the polymer is preferably adjusted to 80 nm/sec or less, more preferably 50 nm/sec or less. In the KrF exposure process, the coating thickness is often 200 nm or greater (i.e., thick film), though depending on a particular purpose, and the rate of dissolution of the polymer used herein is preferably designed to at least 90 nm/sec.

The base polymer may be synthesized by any well-known methods, for example, by performing copolymerization of monomers which are protected with protective groups if necessary, and subsequent deprotection reaction if necessary. The copolymerization reaction is preferably radical or anionic polymerization, though not limited thereto. With respect to the polymerization reaction, reference should be made to WO 2006/121096, JP-A 2008-102383 (U.S. Pat. No. 7,977,027), JP-A 2008-304590 (U.S. Pat. No. 8,343,694), and JP-A 2004-115630.

The polymer preferably has a weight average molecular weight (Mw) of 1,000 to 50,000, and more preferably 2,000 to 20,000, as measured by GPC versus polystyrene standards. A Mw of at least 1,000 avoids the phenomenon that a pattern is rounded at the top so that resolution is reduced and LER is degraded. A Mw of up to 50,000 eliminates the risk of increasing LER in forming a pattern having a line width of up to 100 nm.

Also, the polymer preferably has a narrow dispersity (Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.8. Such a narrow dispersity avoids the risk that a pattern after development has foreign particles left thereon or is degraded in profile.

Other Components

In the chemically amplified negative resist composition, other components such as an organic solvent, quencher, crosslinker, fluorinated polymer, and surfactant may be contained in a suitable combination, if desired. The resist composition thus constructed can form a negative pattern of good profile which is restrained in the dissolution of exposed region in alkaline developer and minimized in top loss, as compared with negative patterns formed from conventional negative resist compositions. The resist film has a high dissolution contrast, high resolution capability, exposure latitude, process adaptability, a good pattern profile after exposure, and minimal proximity bias because of controlled acid diffusion. Because of these advantages, the composition is readily implemented in practice and best suited as a VLSI-forming resist material.

Organic Solvent

The organic solvent used herein is not particularly limited as long as the foregoing and other components are soluble therein. Examples of the organic solvent are described in JP-A 2008-111103, paragraphs [0144]-[0145] (U.S. Pat. No. 7,537,880). Exemplary solvents include ketones such as cyclohexanone, cyclopentanone, methyl-2-n-pentyl ketone and 2-heptanone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, and diacetone alcohol (DAA); ethers such as propylene glycol nonomethyl ether (PGME), ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propylene glycol dimethyl ether, and diethylene glycol dimethyl ether: esters such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butyl acetate, tert-butyl propionate, and propylene glycol mono-tert-butyl ether acetate; and lactones such as γ-butyrolactone, which may be used alone or in admixture.

Of the above organic solvents, 1-ethoxy-2-propanol, PGMEA, ethyl lactate, cyclohexanone, γ-butyrolactone, and a mixture thereof are preferred because the onium salt is fully soluble therein.

The organic solvent is preferably added in an amount of 200 to 12,000 parts, and more preferably 400 to 10,000 parts by weight per 100 parts by weight of the base polymer.

Quencher

The quencher is typically selected from conventional basic compounds. Conventional basic compounds include primary, secondary, and tertiary aliphatic amines, mixed amines, aromatic amines, heterocyclic amines, nitrogen-containing compounds with carboxyl group, nitrogen-containing compounds with sulfonyl group, nitrogen-containing compounds with hydroxyl group, nitrogen-containing compounds with hydroxyphenyl group, alcoholic nitrogen-containing compounds, amide derivatives, imide derivatives, and carbamate derivatives. Also included are primary, secondary, and tertiary amine compounds, specifically amine compounds having a hydroxyl group, ether bond, ester bond, lactone ring, cyano group, or sulfonic acid ester bond as described in JP-A 2008-111103, paragraphs [0146]-[0164], and compounds having a carbamate group as described in JP 3790649. Addition of a basic compound may be effective for further suppressing the diffusion rate of acid in the resist film or correcting the pattern profile.

Onium salts such as sulfonium salts, iodonium salts and ammonium salts of carboxylic acids which are not fluorinated at α-position as described in U.S. Pat. No. 8,795,942 (JP-A 2008-158339) may also be used as the quencher. While an α-fluorinated sulfonic acid, imide acid, and methide acid are necessary to deprotect the acid labile group, an α-non-fluorinated carboxylic acid is released by salt exchange with an α-non-fluorinated onium salt. An α-non-fluorinated carboxylic acid functions as a quencher because it does not induce substantial deprotection reaction.

Examples of the onium salt of α-non-fluorinated carboxylic acid include compounds having the formula (C1). R¹⁰¹—CO₂ ⁻Mq⁺  (C1)

In formula (C1), R¹⁰¹ is hydrogen or a C₁-C₄₀ hydrocarbyl group which may contain a heteroatom, excluding the hydrocarbon group in which the hydrogen atom bonded to the carbon at α-position relative to the carboxyl group is substituted by fluorine or fluoroalkyl.

The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, tert-pentyl, n-hexyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl; cyclic saturated hydrocarbyl groups such as cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexyhnethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, adamantyl, and adamantylmethyl; alkenyl groups such as vinyl, allyl, propenyl, butenyl and hexenyl; cyclic unsaturated aliphatic hydrocarbyl groups such as cyclohexenyl; aryl groups such as phenyl, naphthyl, alkylphenyl groups (e.g., 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 4-ethylphenyl, 4-tert-butylphenyl, 4-n-butylphenyl), dialkylphenyl groups (e.g., 2,4-dimethylphenyl and 2,4,6-triisopropylphenyl), alkylnaphthyl groups (e.g., methylnaphthyl and ethylnaphthyl), dialkylnaphthyl groups (e.g., dimethylnaphthyl and diethylnaphthyl): and aralkyl groups such as benzyl, 1-phenylethyl and 2-phenylethyl.

In these groups, some hydrogen may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, and some carbon may be replaced by a moiety containing a heteroatom such as oxygen, sulfur or nitrogen, so that the group may contain a hydroxyl moiety, cyano moiety, carbonyl moiety, ether bond, thioether bond, ester bond, sulfonic acid ester bond, carbonate bond, lactone ring, sultone ring, carboxylic anhydride, or haloalkyl moiety. Suitable heteroatom-containing hydrocarbyl groups include heteroaryl groups such as thienyl: alkoxyphenyl groups such as 4-hydroxyphenyl, 4-methoxyphenyl, 3-methoxyphenyl, 2-methoxyphenyl, 4-ethoxyphenyl, 4-tert-butoxyphenyl, 3-tert-butoxyphenyl; alkoxynaphthyl groups such as methoxynaphthyl, ethoxynaphthyl, n-propoxynaphthyl and n-butoxynaphthyl; dialkoxynaphthyl groups such as dimethoxynaphthyl and diethoxynaphthyl; and aryloxoalkyl groups, typically 2-aryl-2-oxoethyl groups such as 2-phenyl-2-oxoethyl, 2-(1-naphthyl)-2-oxoethyl and 2-(2-naphthyl)-2-oxoethyl.

In formula (C1), Mq⁺ is an onium cation. The onium cation is preferably selected from sulfonium, iodonium and ammonium cations, more preferably sulfonium and iodonium cations. Exemplary sulfonium cations are as exemplified above for the sulfonium cation having formula (A1-a) wherein p=0. Exemplary iodonium cations are as exemplified above for the iodonium cation having formula (A1-b) wherein q=0. Exemplary ammonium cations are as exemplified above for the ammonium cation having formula (A1-c) wherein r=0.

A sulfonium salt of iodized benzene ring-containing carboxylic acid having the formula (C2) is also useful as the quencher.

In formula (C2), R²⁰¹ is hydroxyl, fluorine, chlorine, bromine, amino, nitro, cyano, or a C₁-C₆ saturated hydrocarbyl, C₁-C₆ saturated hydrocarbyloxy, C₂-C₆ saturated hydrocarbylcarbonyloxy or C₁-C₄ saturated hydrocarbylsulfonyloxy group, in which some or all hydrogen may be substituted by halogen, or —N(R^(201A))—C(═O)—R^(201B), or —N(R^(201A))—C(═O)OR^(201B). R^(201A) is hydrogen or a C₁-C₆ saturated hydrocarbyl group. R^(201B) is a C₁-C₆ saturated hydrocarbyl or C₂-C₈ unsaturated aliphatic hydrocarbyl group.

In formula (C2), x is an integer of 1 to 5, y is an integer of 0 to 3, and z is an integer of 1 to 3. L¹ is a single bond, or a C₁-C₂₀ (z+1)-valent linking group which may contain at least one moiety selected from ether bond, carbonyl moiety, ester bond, amide bond, sultone ring, lactam ring, carbonate moiety, halogen, hydroxyl moiety, and carboxyl moiety. The saturated hydrocarbyl, saturated hydrocarbyloxy, saturated hydrocarbylcarbonyloxy, and saturated hydrocarbylsulfonyloxy groups may be straight, branched or cyclic. Groups R²⁰¹ may be the same or different when y and/or z is 2 or 3.

In formula (C2), R²⁰², R²⁰³ and R²⁰⁴ are each independently halogen, or a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom. The hydrocarbyl group may be saturated or unsaturated and straight, branched or cyclic. Examples thereof include C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₆-C₂₀ aryl, and C₇-C₂₀ aralkyl groups. In these groups, some or all hydrogen may be substituted by hydroxyl, carboxyl, halogen, oxo, cyano, nitro, sultone, sulfone, or sulfonium salt-containing moiety, or some carbon may be replaced by an ether bond, ester bond, carbonyl moiety, amide bond, carbonate moiety or sulfonic acid ester bond. Also R²⁰² and R²⁰³ may bond together to form a ring with the sulfur atom to which they are attached.

Examples of the compound having formula (C2) include those described in U.S. Pat. No. 10,295,904 (JP-A 2017-219836). These compounds exert a sensitizing effect due to remarkable absorption and an acid diffusion controlling effect.

Weak acid betaine compounds are also useful as the quencher. Non-limiting examples thereof are shown below.

Also useful are quenchers of polymer type as described in U.S. Pat. No. 7,598,016 (JP-A 2008-239918). The polymeric quencher segregates at the resist surface after coating and thus enhances the rectangularity of resist pattern. When a protective film is applied as is often the case in the immersion lithography, the polymeric quencher is also effective for preventing a film thickness loss of resist pattern or rounding of pattern top.

When used, the quencher is preferably added in an amount of 0 to 5 parts, more preferably 0 to 4 parts by weight per 100 pants by weight of the base polymer.

Fluorinated Polymer

The negative resist composition may further comprise a fluorinated polymer comprising recurring units having the formula (D1) and recurring units of at least one type selected from recurring units having the formulae (D2), (D3), (D4), and (D5), for the purposes of enhancing contrast, preventing chemical flare of acid upon exposure to high-energy radiation, and suppressing unexpected unnecessary pattern degradation. Notably, recurring units having formulae (D1), (D2), (D3), (D4), and (D5) are also referred to as recurring units (D1), (D2), (D3), (D4), and (D5), respectively. Since the fluorinated polymer also has a surface active function, it can prevent insoluble residues from re-depositing onto the substrate during the development step and is thus effective for preventing development defects.

In formulae (D1) to (D5), R^(C) is each independently hydrogen or methyl. R^(D) is each independently hydrogen, fluorine, methyl or trifluoromethyl. R³⁰¹ is hydrogen or a C₁-C₅ straight or branched hydrocarbyl group in which a heteroatom-containing moiety may intervene in a carbon-carbon bond. R³⁰² is a C₁-C₅ straight or branched hydrocarbyl group in which a heteroatom-containing moiety may intervene in a carbon-carbon bond. R³⁰³, R³⁰⁴, R³⁰⁶ and R³⁰⁷ are each independently hydrogen or a C₁-C₁₀ saturated hydrocarbyl group. R³⁰⁵, R³⁰⁸, R³⁰⁹ and R³¹⁰ are each independently hydrogen, a C₁-C₁₅ hydrocarbyl group, C₁-C₁₅ fluorinated hydrocarbyl group, or an acid labile group. An ether or carbonyl moiety may intervene in a carbon-carbon bond in the hydrocarbyl groups or fluorinated hydrocarbyl groups represented by R³⁰⁵, R³⁰⁸, R³⁰⁹ and R³¹⁰. The subscript g¹ is an integer of 1 to 3, g² is an integer in the range: 0≤g²≤5+2g³−g¹, g³ is 0 or 1, and h is an integer of 1 to 3. X¹ is a single bond, —C(═O)—O— or —C(═O)—NH—. X² is a C₁-C₂₀ (h+1)-valent hydrocarbon group or fluorinated hydrocarbon group.

Suitable C₁-C₅ hydrocarbyl groups represented by R³⁰¹ and R³⁰² include alkyl, alkenyl and alkynyl groups, with the alkyl groups being preferred. Suitable alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, and n-pentyl. In these groups, a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene in a carbon-carbon bond.

In formula (D1), —OR³⁰¹ is preferably a hydrophilic group. In this case, R³⁰¹ is preferably hydrogen or a C₁-C₅ alkyl group in which oxygen intervenes in a carbon-carbon bond.

Examples of the recurring unit (D1) are given below, but not limited thereto. Herein R^(C) is as defined above.

In formula (D1), X¹ is preferably —C(═O)—O— or —C(═O)—NH—. Also preferably R^(C) is methyl. The inclusion of carbonyl in X¹ enhances the ability to trap the acid. A polymer wherein R^(C) is methyl is a rigid polymer having a high glass transition temperature (Tg) which is effective for suppressing acid diffusion. As a result, the stability with time of a resist film is improved, and neither resolution nor pattern profile is degraded.

In formulae (D2) and (D3), examples of the C₁-C₁₀ saturated hydrocarbyl group represented by R³⁰³, R³⁰⁴, R³⁰⁶ and R³⁰⁷ include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, and cyclic saturated hydrocarbyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, adamantyl, and norbornyl. Inter alia, C₁-C₆ saturated hydrocarbyl groups are preferred.

In formulae (D2) to (D5), examples of the C₁-C₁₅ hydrocarbyl group represented by R³⁰⁵, R³⁰⁸, R³⁰⁹ and R³¹⁰ include alkyl, alkenyl and alkynyl groups, with the alkyl groups being preferred. Suitable alkyl groups include n-undecyl, n-dodecyl, tridecyl, tetradecyl and pentadecyl as well as those exemplified above. The fluorinated hydrocarbyl groups correspond to the foregoing hydrocarbyl groups in which some or all carbon-bonded hydrogen atoms are substituted by fluorine atoms.

Examples of the C₁-C₂₀ (h+1)-valent hydrocarbon group or C₁-C₂₀ (h+1)-valent fluorinated hydrocarbon group represented by X² include the foregoing hydrocarbyl groups and fluorinated hydrocarbyl groups, with a number (h) of hydrogen atoms being eliminated.

Examples of the recurring units (D2) to (D5) are given below, but not limited thereto. Herein R^(D) is as defined above.

The recurring unit (D1) is preferably incorporated in an amount of 5 to 85 mol %, more preferably 15 to 80 mol % based on the overall recurring units of the fluorinated polymer. The recurring units (D2) to (D5), which may be used alone or in admixture, are preferably incorporated in an amount of 15 to 95 mol %, more preferably 20 to 85 mol % based on the overall recurring units of the fluorinated polymer.

The fluorinated polymer may comprise additional recurring units as well as the recurring units (D1) to (D5). Suitable additional recurring units include those described in U.S. Pat. No. 9,091,918 (JP-A 2014-177407, paragraphs [0046]-[0078]). When the fluorinated polymer comprises additional recurring units, their content is preferably up to 50 mol % based on the overall recurring units.

The fluorinated polymer may be synthesized by combining suitable monomers optionally protected with a protective group, copolymerizing them in the standard way, and effecting deprotection reaction if necessary. The copolymerization reaction is preferably radical or anionic polymerization though not limited thereto. For the polymerization reaction, reference may be made to JP-A 2004-115630.

The fluorinated polymer should preferably have a Mw of 2,000 to 50,000, and more preferably 3,000 to 20,000. A fluorinated polymer with a Mw of less than 2,000 helps acid diffusion, degrading resolution and detracting from age stability. A polymer with too high Mw has a reduced solubility in solvent, leading to coating defects. The fluorinated polymer preferably has a dispersity (Mw/Mn) of 1.0 to 2.2, more preferably 1.0 to 1.7.

The fluorinated polymer is preferably used in an amount of 0.01 to 30 parts, more preferably 0.1 to 20 parts by weight per 100 parts by weight of the base polymer.

Crosslinker

The resist composition may comprise a crosslinker. Suitable crosslinkers include epoxy compounds, melamine compounds, guanamine compounds, glycoluril compounds and urea compounds having substituted thereon at least one group selected from among methylol, alkoxymethyl and acyloxymethyl groups, isocyanate compounds, azide compounds, and compounds having a double bond such as an alkenyloxy group. These compounds may be used as an additive or introduced into a polymer side chain as a pendant. Hydroxy-containing compounds may also be used as the crosslinker.

Examples of the epoxy compound include tris(2,3-epoxypropyl) isocyanurate, trimethylolmethane triglycidyl ether, trimethylolpropane triglycidyl ether, and triethylolethane triglycidyl ether. Examples of the melamine compound include hexamethylol melamine, hexamethoxymethyl melamine, hexamethylol melamine compounds having 1 to 6 methylol groups methoxymethylated and mixtures thereof, hexamethoxyethyl melamine, hexaacyloxymethyl melamine, hexamethylol melamine compounds having 1 to 6 methylol groups acyloxymethylated and mixtures thereof. Examples of the guanamine compound include tetramethylol guanamine, tetramethoxymethyl guanamine, tetramethylol guanamine compounds having 1 to 4 methylol groups methoxymethylated and mixtures thereof, tetramethoxyethyl guanamine, tetraacyloxyguanamine, tetramethylol guanamine compounds having 1 to 4 methylol groups acyloxymethylated and mixtures thereof. Examples of the glycoluril compound include tetramethylol glycoluril, tetramethoxyglycoluril, tetramethoxymethyl glycoluril, tetramethylol glycoluril compounds having 1 to 4 methylol groups methoxymethylated and mixtures thereof, tetramethylol glycoluril compounds having 1 to 4 methylol groups acyloxymethylated and mixtures thereof. Examples of the urea compound include tetramethylolurea, tetramethoxymethyl urea, tetramethylol urea compounds having 1 to 4 methylol groups methoxymethylated and mixtures thereof, and tetramethoxyethyl urea.

Suitable isocyanate compounds include tolylene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate and cyclohexane diisocyanate. Suitable azide compounds include 1,1′-biphenyl-4,4′-bisazide, 4,4′-methylidenebisazide, and 4,4′-oxybisazide. Examples of the alkenyloxy group-containing compound include ethylene glycol divinyl ether, triethylene glycol divinyl ether, 1,2-propanediol divinyl ether, 1,4-butanediol divinyl ether, tetramethylene glycol divinyl ether, neopentyl glycol divinyl ether, trimethylol propane trivinyl ether, hexanediol divinyl ether, 1,4-cyclohexanediol divinyl ether, pentaerythritol trivinyl ether, pentaerythritol tetravinyl ether, sorbitol tetravinyl ether, sorbitol pentavinyl ether, and trimethylol propane trivinyl ether.

When the resist composition contains a crosslinker, the crosslinker is preferably added in an amount of 2 to 50 parts, more preferably 5 to 30 parts by weight per 100 parts by weight of the base polymer. As long as the amount of crosslinker is in the range, the risk of bridging pattern features to reduce resolution is minimized. The crosslinker may be used alone or in admixture. It is noted that the crosslinker is not essential because the inventive resist composition is fully suppressed in the dissolution of exposed region in alkaline developer even when the composition does not contain the crosslinker.

Surfactant

The resist composition may contain any conventional surfactants for facilitating to coat the composition to the substrate. Since a number of surfactants are known in the art, for example, PF-636 (Onmova Solutions), FC-4430 (3M), and those described in JP-A 2004-115630, any suitable one may be chosen therefrom. The amount of surfactant added is preferably 0 to 5 parts by weight per 100 parts by weight of the base polymer.

Pattern Forming Process

A further embodiment of the invention is a process for forming a resist pattern comprising the steps of applying the chemically amplified negative resist composition onto a substrate to form a resist film thereon, exposing patternwise the resist film to high-energy radiation, and developing the exposed resist film in an alkaline developer.

The resist composition is first applied onto a substrate on which an integrated circuit is to be formed (e.g., Si, SiO₂, SiN, SiON, TiN, WSi, BPSG, SOG, or organic antireflective coating) or a substrate on which a mask circuit is to be formed (e.g., Cr, CrO, CrON, MoSi₂, or SiO₂) by a suitable coating technique such as spin coating. The coating is prebaked on a hot plate at a temperature of preferably 60 to 150° C. for 1 to 20 minutes, more preferably at 80 to 140° C. for 1 to 10 minutes. The resulting resist film is generally 0.03 to 2 μm thick.

The resist film is then exposed to a desired pattern of high-energy radiation such as UV, deep-UV, excimer laser radiation (KrF, ArF, etc.), EUV, x-ray, γ-ray, synchrotron radiation or EB. Exposure using KrF excimer laser, EUV or EB is preferred.

When UV, deep-UV, excimer laser, EUV, x-ray, γ-ray or synchrotron radiation is used as the high-energy radiation, the resist film is exposed thereto through a mask having a desired pattern in a dose of preferably 1 to 200 mJ/cm², more preferably 10 to 100 mJ/cm². When EB is used as the high-energy radiation, the resist film is exposed thereto directly in a dose of preferably 1 to 300 μC/cm², more preferably 10 to 200 μC/cm².

After the exposure, the resist film may be baked (PEB) on a hotplate preferably at 60 to 150° C. for 1 to 20 minutes, more preferably at 80 to 140° C. for 1 to 10 minutes.

After the exposure or PEB, the resist film is developed in a developer in the form of an aqueous alkaline solution for preferably 0.1 to 3 minutes, more preferably 0.5 to 2 minutes by conventional techniques such as dip, puddle and spray techniques. A typical developer is a 0.1 to 5 wt %, preferably 2 to 3 wt % aqueous solution of tetramethylammonium hydroxide (TMAH) or another alkali. In this way, the desired pattern is formed on the substrate.

After the resist film is formed, rinsing with deionized water, known as post-soaking, may be carried out to extract the acid generator or the like from the film surface and to wash away particles. The resist film may also be rinsed (post-soaking) for removing any residual water on the film after exposure.

EXAMPLES

Examples of the invention are given below by way of illustration and not by way of limitation. The abbreviation “pbw” is parts by weight. For all polymers, Mw and Mn are determined by GPC versus polystyrene standards using THF or DMF solvent. THF is tetrahydrofuran, DMF is dimethylformamide, MIBK is methyl isobutyl ketone, and PGMEA is propylene glycol monomethyl ether acetate. Analysis is made by IR spectroscopy, NMR spectroscopy, and time-of-flight mass spectrometry (TOF-MS) using analytic instruments as shown below.

IR: NICOLET 6700 by Thermo Fisher Scientific Inc.

¹H-, ¹³C- and ¹⁹F-NMR: ECA-500 by JEOL Ltd.

MALDI TOF-MS: S3000 by JEOL Ltd.

[1] Synthesis of Onium Salts Example 1-1

Synthesis of PAG-1

(1) Synthesis of Intermediate In-1

In nitrogen atmosphere, a flask was charged with 145.9 g (6 equivalents relative to M-1) of magnesium, 3,000 g of THF, and chloromethane, from which a Grignard reagent was prepared. A solution of 210 g of M-1 in 500 g of THF was added dropwise to the flask at a temperature of 40-55° C. (internal temperature, hereinafter). At the end of addition, the solution was aged at an internal temperature of 50° C. for 3 hours. After aging, the reaction system was cooled, whereupon an aqueous solution of 600 g of ammonium chloride and 1,800 g of 3.0 wt % hydrochloric acid aqueous solution was added dropwise to quench the reaction. This was followed by extraction with 2,000 mL of ethyl acetate, ordinary aqueous work-up, solvent distillation, and recrystallization from hexane. There was obtained an Intermediate In-1 as white crystals (amount 172.2 g, yield 82%).

(2) Synthesis of Intermediate In-2

In nitrogen atmosphere, a flask was charged with 105.1 g of Intermediate In-1, 76.0 g of potassium carbonate, 7.5 g of sodium iodide, and 350 g of DMF, which were stirred at an internal temperature of 50° C. for 30 minutes. To the flask, a solution of 82.0 g of isopropyl chloroacetate in 80 g of DMF was added dropwise. At the end of addition, the solution was aged for 18 hours while keeping the internal temperature of 50° C. After aging, the reaction system was cooled, whereupon a solution of 150.4 g of 20 wt % hydrochloric acid aqueous solution and 500 g of water was added dropwise to quench the reaction. Thereafter, 500 mL of ethyl acetate and 250 mL of toluene were added to the solution to extract the end compound, followed by ordinary aqueous work-up, solvent distillation, and recrystallization from hexane. There was obtained an Intermediate In-2 as white crystals (amount 136.1 g, yield 88%).

(3) Synthesis of Intermediate In-3

In nitrogen atmosphere, a flask was charged with 136.1 g of Intermediate In-2 and 550 g of 1,4-dioxane and cooled in an ice bath. To the flask, 84.2 g of 25 wt % sodium hydroxide aqueous solution was added dropwise. At the end of addition, the solution was heated at an internal temperature of 40° C. and aged for 12 hours. After aging, the reaction solution was ice cooled, whereupon 100.7 g of 20 wt % hydrochloric acid aqueous solution was added dropwise to quench the reaction. This was followed by extraction with 1,000 mL of ethyl acetate, ordinary aqueous work-up, solvent distillation, and recrystallization from hexane. There was obtained an Intermediate In-3 as white crystals (amount 85.5 g, yield 73%).

(4) Synthesis of Intermediate In-4

In nitrogen atmosphere, a flask was charged with 26.8 g of Intermediate In-3, 38.0 g of benzyltrimethylammonium 2-isopropyl-5-methyl-4-hydroxybenzenesulfonate, 1.2 g of N,N-dimethyl-4-aminopyridine (DMAP), and 200 g of methylene chloride and cooled in an ice bath. To the flask, 23.0 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride in powder form was added while keeping the internal temperature below 20° C. At the end of addition, the reaction mixture was heated at room temperature and aged for 12 hours. After aging, water was added to quench the reaction. This was followed by ordinary aqueous work-up, solvent distillation, and azeotropic dehydration with 100 g of MIBK. There was obtained an Intermediate In-4 as oily matter (amount 50.3 g, yield 80%).

(5) Synthesis of PAG-1

In nitrogen atmosphere, 15.0 g of Intermediate In-4, 7.1 g of triphenylsulfonium chloride, 75 g of MIBK, 25 g of methylene chloride, and 60 g of water were combined and stirred for 30 minutes. The organic layer was taken out, washed with water, and concentrated under reduced pressure. Then diisopropyl ether was added to the concentrate for recrystallization. There was obtained the target compound PAG-1 as white crystals (amount 14.1 g, yield 80%).

PAG-1 was analyzed by IR spectroscopy and TOF-MS, with the data shown below. FIG. 1 is the ¹H-NMR/DMSO-d₆ spectrum of PAG-1.

IR (D-ATR): ν=3381, 3061, 2971, 2929, 2869, 1771, 1593, 1476, 1446, 1375, 1324, 1294, 1257, 1217, 1168, 1087, 1041, 997, 964.923, 863, 835, 782, 750, 711, 683, 644, 622, 561, 502, 433 cm⁻¹

MALDI TOF-MS

Positive M⁺ 263 (corresponding to C₁₈H₁₅S⁺)

Negative M⁻ 479 (corresponding to C₂₄H₃₁O₈S⁻)

Examples 1-2 to 1-14, Comparative Examples 1-1 to 1-3 and Reference Examples 1-1 to 1-2 Synthesis of PAG-2 to PAG-14, cPAG-1 to cPAG-3, PAG-A, and PAG-B

Onium salts PAG-2 to PAG-14 and cPAG-1 to cPAG-3 were synthesized by well-known organic synthesis methods. Also, PAG-A and PAG-B which were used in a complementary manner for improving solubility in the developer were synthesized. PAG-2 to PAG-14, cPAG-1 to cPAG-3, PAG-A and PAG-B have the structure shown below.

[2] Synthesis of Polymers Synthesis Example 1

Synthesis of Polymer P-1

A 3-L flask was charged with 301.8 g of 4-acetoxystyrene, 190.7 g of acenaphthylene, 513 g of a 55.8 wt % solution of 4-ethenyl-α,α-dimethylbenzenemethanol in PGMEA, and 814 g of toluene solvent. The reactor was cooled at −70° C. in nitrogen atmosphere, after which vacuum pumping and nitrogen flow were repeated three times. The reactor was warmed up to room temperature, whereupon 40.5 g of 2,2′-azobis(2,4-dimethylvaleronitrile) (V-65 by Fuji Film Wako Pure Chemical Industries, Ltd.) was added as polymerization initiator. The reactor was heated at 45° C., whereupon reaction ran for 20 hours. The temperature was raised to 55° C., at which reaction ran for a further 20 hours. The reaction solution was concentrated to ½ in volume and poured into 7,000 g of hexane for precipitation. The precipitate was filtered and dried in vacuum at 40° C., yielding a white solid.

The white solid was dissolved in a mixture of 300 g of methanol and 900 g of THF again, to which 170.5 g of 2-ethanolamine was added. The solution was kept at 60° C. for 3 hours for deprotection reaction to take place. The reaction solution was concentrated, and dissolved in 1,500 g of ethyl acetate. The solution was subjected to once neutralization/separatory washing with a mixture of 450 g water and 85.3 g acetic acid, once separatory washing with a mixture of 450 g water and 113.7 g pyridine, and 4 times separatory washing with 450 g water. Thereafter, the upper layer or ethyl acetate solution was concentrated, dissolved in 850 g of acetone, and poured into 15 L of water for precipitation. The precipitate was filtered and dried in vacuum at 50° C. for 40 hours, yielding 349.4 g of a polymer as white solid. The polymer, designated Polymer P-1, was analyzed by ¹H-NMR, ¹³C-NMR, and GPC (THF solvent), with the results shown below.

Synthesis Example 2

Synthesis of Polymer P-6

In nitrogen atmosphere, 890 g of 50.0 wt % PGMEA solution of 4-hydroxystyrene, 47.7 g of acenaphthylene, 310 g of 54.7 wt % PGMEA solution of 4-(2-hydroxy-2-propyl)styrene, 87.0 g of triphenylsulfonium 1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate, 96.1 g of dimethyl 2,2′-azobis(2-methylpropionate) (V-601 by Fuji Film Wako Pure Chemical Industries, Ltd.), and 360 g of γ-butyrolactone and 220 g of PGMEA as solvent were fed into a 3000-mL dropping cylinder to form a monomer solution. In nitrogen atmosphere, a 5000-mL polymerization flask was charged with 580 g of γ-butyrolactone, which was heated at 80° C. The monomer solution was added dropwise from the dropping cylinder to the flask over 4 hours. After the completion of dropwise addition, the polymerization solution was continuously stirred for 18 hours while maintaining its temperature at 80° C. The polymerization solution was cooled to room temperature and added dropwise to 22.5 kg of diisopropyl ether whereupon a white solid precipitated. Diisopropyl ether was decanted off and the white solid precipitate was dissolved in 2,250 g of acetone. The acetone solution was added dropwise to 22.5 kg of diisopropyl ether whereupon a white solid precipitated. The white solid precipitate was collected by filtration and dissolved in 2,250 g of acetone again. The acetone solution was added dropwise to 22.5 kg of water whereupon a white solid precipitated. The white solid was collected by filtration and dried at 40° C. for 40 hours, obtaining 700 g of a polymer.

The polymer designated Polymer P-6 was analyzed by ¹H-NMR, ¹³C-NMR and GPC (DMF solvent), with the results shown below.

Synthesis Examples 3 to 24

Synthesis of Polymers P-2 to P-5, P-7 to P-24

Polymers P-2 to P-5, P-7 to P-24 were synthesized by the same procedures as Synthesis Example 1 or 2 except that the type and amount (mol %) of monomers were changed. For Polymers P-1 to P-24, the type and molar ratio of monomers are tabulated in Table 1. Notably, Mw of Polymers P-1 to P-5, P-15, P-16, P-18, P-20 to P-23 was measured by GPC using THF solvent, and Mw of Polymers P-6 to P-14, P-17, P-19, and P-24 was measured by GPC using DMF solvent.

TABLE 1 Proportion Proportion Proportion Proportion Proportion Polymer Unit 1 (mol %) Unit 2 (mol %) Unit 3 (mol %) Unit 4 (mol %) Unit 5 (mol %) Mw Mw/Mn P-1 A-1 60.0 B-1 10.0 C-1 30.0 — — — — 3,700 1.62 P-2 A-1 70.0 B-1 7.0 C-2 23.0 — — — — 3,600 1.63 P-3 A-1 70.0 B-1 10.0 C-3 20.0 — — — — 3,900 1.65 P-4 A-1 70.0 B-1 10.0 C-4 20.0 — — — — 4,200 1.64 P-5 A-1 55.0 B-2 10.0 C-1 35.0 — — — — 4,000 1.63 P-6 A-1 66.0 B-1 9.0 C-1 21.5 E-1 3.5 — — 13,000 1.62 P-7 A-1 60.0 B-1 4.0 C-1 24.0 E-1 12.0 — — 15,000 1.65 P-8 A-1 67.0 B-1 10.0 C-1 18.5 E-2 4.5 — — 14,000 1.63 P-9 A-1 67.0 B-1 9.3 C-1 20.0 E-3 3.7 — — 13,500 1.63 P-10 A-1 67.3 B-1 10.0 C-1 17.5 E-4 5.2 — — 13,200 1.64 P-11 A-1 64.1 B-1 9.5 C-1 22.0 E-5 4.4 — — 12,800 1.62 P-12 A-1 64.0 B-1 10.0 C-1 22.8 E-6 3.2 — — 13,500 1.63 P-13 A-1 62.0 B-2 10.0 C-1 24.3 E-1 3.7 — — 12,400 1.66 P-14 A-2 60.5 B-3 10.0 C-1 24.4 E-2 5.1 — — 12,300 1.65 P-15 A-1 80.0 C-1 20.0 — — — — — — 4,200 1.69 P-16 A-1 80.0 B-1 5.0 C-1 15.0 — — — — 4,300 1.67 P-17 A-1 80.0 B-1 2.5 C-1 15.0 E-1 2.5 — — 12,100 1.69 P-18 A-2 50.0 C-1 30.0 D-1 20.0 — — — — 4,600 1.67 P-19 A-2 50.0 B-1 2.5 C-1 30.0 D-1 15.0 E-1 2.5 12,700 1.73 P-20 A-2 50.0 C-1 30.0 D-2 20.0 — — — — 5,400 1.72 P-21 A-2 50.0 C-1 30.0 D-3 20.0 — — — — 6,100 1.73 P-22 A-2 50.0 C-1 30.0 D-4 20.0 — — — — 7,000 1.76 P-23 A-1 67.5 B-1 2.5 C-1 30.0 — — — — 4,100 1.65 P-24 A-1 57.5 B-1 2.5 C-1 30.0 E-5 10 — — 11,000 1.65

The structure of each recurring unit is shown below.

[3] Preparation of Negative Resist Compositions Examples 2-1 to 2-42 and Comparative Examples 2-1 to 2-9

Negative resist compositions (R-1 to R-42) and comparative negative resist compositions (cR-1 to cR-9) were prepared by dissolving selected components in an organic solvent in accordance with the formulation shown in Tables 2 to 5, and filtering the solution through a UPE filter and/or nylon filter with a pore size of 0.02 μm. The organic solvent was a mixture of 1,204 pbw of PGMEA, 1,204 pbw of ethyl lactate, and 1,606 pbw of propylene glycol monomethyl ether. To some compositions, a fluorinated polymer (FP-1 to FP-3) as additive and tetramethoxymethylglycoluril (TMGU) as crosslinker were added. Also some compositions contained surfactant PolyFox PF-636 (Omnova Solutions).

TABLE 2 Acid Base Base Resist gentrator polymer 1 polymer 2 Quencher Additive Surfactant composition (pbw) (pbw) (pbw) (pbw) (pbw) (pbw) Example 2-1 R-1 PAG-1 (8) P-1 — Q-1 FP-1 PF-636 PAG-A (2) (80) (3.0) (3) (0.075) 2-2 R-2 PAG-1 (8) P-1 — Q-1 FP-2 PF-636 PAG-A (2) (80) (3.0) (3) (0.075) 2-3 R-3 PAG-1 (8) P-1 — Q-1 FP-3 PF-636 PAG-A(2) (80) (3.0) (3) (0.075) 2-4 R-4 PAG-1 (8) P-1 — Q-1 TMGU PF-636 PAG-A (2) (80) (3.0) (8.1) (0.075) 2-5 R-5 PAG-1 (8) P-1 — Q-1 — — PAG-A (2) (80) (3.0) 2-6 R-6 PAG-1 P-1 — Q-1 — — (10) (80) (3.0) 2-7 R-7 PAG-2 P-2 — Q-1 — — (10) (80) (3.0) 2-8 R-8 PAG-3 P-3 — Q-1 — — (10) (80) (3.0) 2-9 R-9 PAG-4 P-4 — Q-1 — — (10) (80) (3.0) 2-10 R-10 PAG-5 P-5 — Q-1 — — (10) (80) (3.0) 2-11 R-11 PAG-6 P-6 — Q-2 — — (10) (80) (3.0) 2-12 R-12 PAG-7 P-6 — Q-3 — — (10) (80) (3.0) 2-13 R-13 PAG-1 P-6 P-1 Q-1 — — (5) (40) (40) (3.0) 2-14 R-14 PAG-1 P-6 P-2 Q-1 — — (5) (40) (40) (3.0) 2-15 R-15 PAG-1 (5) P-6 P-1 Q-1 — — PAG-A (2) (40) (40) (3.0) 2-16 R-16 PAG-1 (5) P-6 P-1 Q-1 — — PAG-B (2) (40) (40) (3.0)

TABLE 3 Acid Base Base Resist generator polymer 1 polymer 2 Quencher Additive Surfactant composition (pbw) (pbw) (pbw) (pbw) (pbw) (pbw) Example 2-17 R-17 PAG-1 P-7 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-18 R-18 PAG-1 P-8 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-19 R-19 PAG-2 P-9 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-20 R-20 PAG-2 P-10 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-21 R-21 PAG-3 P-11 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-22 R-22 PAG-8 P42 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-23 R-23 PAG-8 P-13 — Q-1 — PF-636 (5) (80) (30) (0.075) 2-24 R-24 PAG-9 P-14 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-25 R-25 PAG-9 P-15 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-26 R-26 PAG-10 P46 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-27 R-27 PAG-10 P-17 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-28 R-28 PAG-11 P-18 — Q-2 — PF-636 (10) (80) (3.0) (0.075) 2-29 R-29 PAG-11 P-19 — Q-3 — PF-636 (5) (80) (3.0) (0.075) 2-30 R-30 PAG-12 P-20 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-31 R-31 PAG-12 P-21 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-32 R-32 PAG-13 P22 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-33 R-33 PAG-13 P-23 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-34 R-34 PAG-14 P-23 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-35 R-35 PAG-14 P-24 — Q-1 — PF-636 (5) (80) (3.0) (0.075)

TABLE 4 Acid Base Base Resist generator polymer 1 polymer 2 Quencher Additive Surfactant composition (pbw) (pbw) (pbw) (pbw) (pbw) (pbw) Example 2-36 R-36 PAG-1 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-37 R-37 PAG-2 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-38 R-38 PAG-10 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-34 R-39 PAG-11 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-40 R-40 PAG-12 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-41 R-41 PAG-13 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-42 R-42 PAG-14 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075)

TABLE 5 Acid Base Base Resist generator polymer 1 polymer 2 Quencher Additive Surfactant composition (pbw) (pbw) (pbw) (pbw) (pbw) (pbw) Comparative 2-1 cR-1 cPAG-1 (5) P-6 — Q-1 — — Example PAG-A(2) (80) (3.0) 2-2 cR-2 cPAG-1 P-6 — Q-1 — — (10) (80) (3.0) 2-3 cR-3 cPAG-2 P-6 P-1 Q-1 — — (5) (40) (40) (3.0) 2-4 cR-4 cPAG-1 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-5 cR-5 cPAG-2 P-7 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-6 cR-6 cPAG-3 P-7 — Q-1 — PF-636 (5) (80) (3.0) (0.075) 2-7 cR-7 cPAG-1 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-8 cR-8 cPAG-2 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075) 2-9 cR-9 cPAG-3 P-1 — Q-1 — PF-636 (10) (80) (3.0) (0.075)

The fluorinated polymers FP-1 to FP-3 and quenchers Q-1 to Q-3 in Tables 2 to 5 are identified below.

[4] EB Lithography Test Examples 3-1 to 3-16 and Comparative Examples 3-1 to 3-3

An antireflective coating solution (DUV-42, Nissan Chemical Corp.) was coated on a silicon substrate and baked at 200° C. for 60 seconds to form an ARC of 61 nm thick. Each of the resist compositions (R-1 to R-16, cR-1 to cR-3) was spin coated on the ARC and prebaked on a hotplate at 100° C. for 60 seconds to form a resist film of 45 nm thick. Using an EB lithography system ELS-F125 (Elionix Co., Ltd., accelerating voltage 125 kV), the resist film was exposed to a dot pattern of EB having a size of 26 nm and a pitch of 52 nm (on-wafer) while changing the dose from 50 μC/cm² at a step of 1 μC/cm². The resist film was baked (PEB) at the temperature shown in Table 6 for 60 seconds and then puddle developed in a 2.38 wt % TMAH aqueous solution for 30 seconds. The resist film was rinsed with deionized water and spin dried, yielding a negative resist pattern or dot pattern. The dot pattern was observed under CD-SEM S9380 (Hitachi High Technologies Corp.) whereupon sensitivity, exposure latitude (EL), and CDU were evaluated by the following methods. The results are shown in Table 6.

Evaluation of Sensitivity

The optimum dose Eop (μC/cm²) which provided a dot pattern with a size of 26 nm and a pitch of 52 mm was determined as an index of sensitivity. A smaller dose value indicates a higher sensitivity.

Evaluation of EL

The exposure dose which provided a dot pattern with a size of 26 nm±10% (i.e., 23 nm to 29 nm) was determined. EL (%) is calculated from the exposure doses according to the following equation: EL (%)=(|E1−E2|/Eop)×100 wherein E1 is an optimum exposure dose which provides a dot pattern with a size of 23 nm and a pitch of 52 nm, E2 is an optimum exposure dose which provides a dot pattern with a size of 29 nm and a pitch of 52 nm, and Eop is an optimum exposure dose which provides a dot pattern with a size of 26 nm and a pitch of 52 nm. A larger value indicates better EL. Evaluation of CDU

For the dot pattern printed by exposure at the optimum dose Eop, the size was measured at 10 areas subject to an identical dose of shot (9 dots per area), from which a 3-fold value (3σ) of standard deviation (σ) was determined and reported as CDU. A smaller value of 3σ indicates a dot pattern having improved CDU.

TABLE 6 Resist PEB temp. Sensitivity EL CDU composition (° C.) (μC/cm²) (%) (nm) Example 3-1 R-1 100 145 15.4 3.4 3-1 R-2 100 149 14.5 3.4 3-3 R-3 100 150 15.1 3.6 3-4 R-4 100 160 16.5 4.0 3-5 R-5 100 140 15.0 3.5 3-6 R-6 100 146 14.1 3.6 3-7 R-7 100 159 14.2 4 3-8 R-8 100 165 15.1 3.8 3-9 R-9 100 147 11.9 3.6 3-10 R-10 100 158 12.7 3.8 3-11 R-11 100 146 14.8 4.0 3-12 R-12 100 148 14.7 3.4 3-13 R-13 100 154 15.0 3.6 3-14 R-14 100 161 15.6 3.5 3-15 R-15 100 145 14.7 3.4 3-16 R-16 100 165 15.2 3.7 Comparative 3-1 cR-1 100 195 10.1 4.7 Example 3-2 cR-2 100 220 9.5 4.4 3-3 cR-3 100 210 10.1 4.3

[5] EUV Lithography Test Examples 4-1 to 4-19 and Comparative Examples 4-1 to 4-3

Each of the resist compositions (R-17 to R-35, cR-4 to cR-6) was spin coated on a silicon substrate having a 20-nm coating of silicon-containing spin-on hard mask SHB-A940 (Shin-Etsu Chemical Co., Ltd., Si content 43 wt %) and prebaked on a hotplate at 105° C. for 60 seconds to form a resist film of 50 nm thick. Using an EUV scanner NXE3300 (ASML, NA 0.33, a 0.9/0.5, quadrupole illumination), the resist film was exposed to EUV. The resist film was baked (PEB) on a hotplate at the temperature shown in Table 7 for 60 seconds and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds. The unexposed region was dissolved in the developer, yielding a negative resist pattern in the form of a LS pattern having a space width of 23 nm and a pitch of 46 m. The LS pattern was observed under CD-SEM (CG-5000 by Hitachi High Technologies Corp.). The resist pattern was evaluated as follows, with the results shown in Table 7.

Evaluation of Sensitivity

The optimum dose Eop (mJ/cm²) which provided a LS pattern with a space width of 23 nm and a pitch of 46 nm was determined as an index of sensitivity.

Evaluation of LWR

A LS pattern was formed by exposure in the optimum dose Eop. The space width was measured at longitudinally spaced apart 10 points, from which a 3-old value (3c) of standard deviation (a) was determined and reported as LWR. A smaller value of 30 indicates a pattern having a lower roughness and more uniform space width.

Evaluation of Profile

The profile of patterns printed at the optimum dose Eop was compared. A resist film providing a pattern of rectangular profile and perpendicular sidewall is evaluated good. A resist film providing a pattern of tapered profile with remarkably inclined sidewall or top-rounded profile due to top loss is evaluated NG.

TABLE 7 Resist PEB temp. Sensitivity LWR Pattern composition (° C.) (mJ/cm²) (nm) profile Example 4-1 R-17 100 21 4.0 good 4-2 R-18 100 22 3.4 good 4-3 R-19 100 22 3.8 good 4-4 R-20 100 21 3.7 good 4-5 R-21 100 27 3.7 good 4-6 R-22 100 23 3.9 good 4-7 R-23 100 23 4.1 good 4-8 R-24 100 27 4.0 good 4-9 R-25 100 25 3.6 good 4-10 R-26 100 26 3.5 good 4-11 R-27 100 22 3.5 good 4-12 R-28 100 19 3.8 good 4-13 R-29 100 18 3.6 good 4-14 R-30 100 22 3.4 good 4-15 R-31 100 24 3.2 good 4-16 R-32 100 24 3.4 good 4-17 R-33 100 23 3.3 good 4-18 R-34 100 22 3.7 good 4-19 R-35 100 25 3.8 good Comparative 4-1 cR-4 100 31 5.1 NG Example 4-2 cR-5 100 32 5.2 NG 4-3 cR-6 100 29 4.9 NG

[6] Evaluation of Remaining Film in Exposed Region Examples 5-1 to 5-7 and Comparative Examples 5-1 to 5-3

Each of the resist compositions (R-36 to R-42, cR-7 to cR-9) was spin coated on a silicon substrate having a 20-nm coating of silicon-containing spin-on hard mask SHB-A940 (Shin-Etsu Chemical Co., Ltd., Si content 43 wt %) and prebaked on a hotplate at 105° C. for 60 seconds to form a resist film of 50 nm thick. By means of KrF excimer laser scanner S206D (Nikon Corp., NA=0.82, conventional illumination), the resist film was exposed in a varying dose. The resist film was baked (PEB) on a hotplate at the temperature shown in Table 8 for 60 seconds and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds.

The thickness of the resist film in areas of different doses was measured by an ellipsometric film thickness measurement system RE-3100 (Screen Semiconductor Solutions Co., Ltd.). The thickness of film in each dose area divided by the thickness of film as coated is computed and reported as a film retention (%). FIG. 2 is a graph which is drawn by plotting the dose on the abscissa and the film retention on the ordinate. As is evident from FIG. 2 , the chemically amplified negative resist composition to be developed in an alkaline developer shows the tendency that the resist film retention ramps with an increasing dose. The film retention is eventually saturated. The film retention as saturated (the value of arrow A in FIG. 2 ) is shown as “saturation film retention” in Table 8. A large value of saturation film retention indicates that the dissolution of exposed region in alkaline developer is suppressed. Film retention (%)=[thickness (nm) of film in each dose area]/[thickness (=50 nm) of film as coated]×100

TABLE 8 PEB Saturation film Resist temperature retention composition (° C.) (%) Example 5-1 R-36 100 91 5-2 R-37 100 93 5-3 R-38 100 94 5-4 R-39 100 97 5-5 R-40 100 89 5-6 R-41 100 94 5-7 R-42 100 96 Comparative 5-1 cR-7 100 84 Example 5-2 cR-8 100 76 5-3 cR-9 100 79

As is evident from Tables 6 and 7, the chemically amplified negative resist composition is excellent in sensitivity, LWR, CDU and pattern profile when processed by the EB lithography and EUV lithography. As is evident from Table 8, the chemically amplified negative resist composition shows improved saturation film retention, i.e., controlled dissolution of exposed region in alkaline developer, when processed by the KrF lithography.

Japanese Patent Application No. 2020-097357 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

The invention claimed is:
 1. An onium salt containing an anion having the formula (A1) and a cation selected from the formulae (A1-a) to (A1-c):

wherein k¹ is an integer of 0 to 2, k² is an integer meeting 0≤k²≤4+4k¹, m¹ is an integer of 1 to 4, m² is each independently an integer of 0 to 4, n is each independently an integer of 0 to 10, p is an integer of 0 to 3, q is an integer of 0 to 2, r is an integer of 0 to 4, R^(a1) and R^(a2) are each independently hydrogen or a C₁-C₁₀ hydrocarbyl group in which some or all hydrogen atoms may be substituted by halogen and a constituent —CH₂— may be replaced by —O— or —C(═O)—, both R^(a1) and R^(a2) are not hydrogen at the same time, R^(a1) and R^(a2) may bond together to form an aliphatic ring with the carbon atom to which they are attached, R^(a3) is each independently a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, when n is 2 or more, R^(a3) may be the same or different and two or more R^(a3) may bond together to form a ring with the atom on W² to which they are attached, R^(a4) is a C₁-C₁₀ hydrocarbyl group which may contain a heteroatom, W¹ is a C₃-C₂₀ (n+2)-valent group having a mono- or polycyclic structure, W² is each independently a C₃-C₂₀ (m²+n+1)-valent group having a mono- or polycyclic structure, in which a constituent —CH₂— may be replaced by —O—, —S— or —C(═O)—, L^(a1) and L^(a2) are each independently a single bond, ether bond, ester bond, sulfonate bond, carbonate bond or carbamate bond, X^(L) is a single bond or a C₁-C₄₀ hydrocarbylene group which may contain a heteroatom, R¹, R² and R³ are each independently a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, any two of two R¹ and W² may bond together to form a ring with the sulfur atom in the formula, any two of three R³ and W² may bond together to form a ring with the nitrogen atom in the formula.
 2. The onium salt of claim 1 wherein the anion having formula (A1) has the formula (A2):

wherein L^(a1), L^(a2), X^(L), R^(a1) to R^(a4), k¹, k², m¹ and n are as defined above, s is an integer of 0 to
 2. 3. The onium salt of claim 2 wherein the anion having formula (A2) has the formula (A3):

wherein L^(a1), X^(L), R^(a1) to R^(a4), k¹, k², m¹, n and s are as defined above.
 4. The onium salt of claim 1 wherein R^(a1) and R² are each independently hydrogen or a C₁-C₁₀ hydrocarbyl group in which a constituent —CH₂— may be replaced by —O— or —C(═O)—, both R^(a1) and R^(a2) are not hydrogen at the same time, R^(a1) and R^(a2) may bond together to form an aliphatic ring with the carbon atom to which they are attached.
 5. The onium salt of claim 1 wherein L^(a1) and L^(a2) are each independently a single bond, ether bond or ester bond.
 6. A photoacid generator comprising the onium salt of claim
 1. 7. A chemically amplified negative resist composition comprising the photoacid generator of claim
 6. 8. The chemically amplified negative resist composition of claim 7, further comprising a base polymer comprising recurring units having the formula (B1) and recurring units having the formula (B2):

wherein R^(A) is hydrogen, fluorine, methyl or trifluoromethyl, R¹¹ and R¹² are each independently halogen, an optionally halogenated C₁-C₆ saturated hydrocarbyl group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, or optionally halogenated C₁-C₆ saturated hydrocarbyloxy group, A¹ and A² are each independently a single bond or a C₁-C₁₀ saturated hydrocarbylene group in which a constituent —CH₂— may be replaced by —O—, W¹¹ is hydrogen, a C₁-C₁₀ aliphatic hydrocarbyl group, or an optionally substituted aryl group, a constituent —CH₂— in the aliphatic hydrocarbyl group may be replaced by —O—, —C(═O)—, —O—C(═O)— or —C(═O)—O—, R^(x) and R^(y) are each independently hydrogen, or a C₁-C₁₅ saturated hydrocarbyl group which may be substituted with a hydroxyl or saturated hydrocarbyloxy moiety, or an optionally substituted aryl group, both R^(x) and R^(y) are not hydrogen at the same time, R^(x) and R^(y) may bond together to form a ring with the carbon atom to which they are attached, t¹ and t² are each independently 0 or 1, x¹ and x² are each independently an integer of 0 to 2, a¹ and b¹ are each independently an integer of 1 to 3, a² is an integer meeting 0≤a²≤5+2x¹−a¹, and b² is an integer meeting 0≤b²≤5+2x²−b¹.
 9. The chemically amplified negative resist composition of claim 8, wherein the recurring units of formula (B1) have the formula (B1-1) and the recurring units of formula (B2) have the formula (B2-1):

wherein R^(A), R¹¹, R¹², R^(x), R^(y), W¹¹, a¹ and b¹ are as defined above.
 10. The chemically amplified negative resist composition of claim 8, wherein the base polymer further comprises recurring units of at least one type selected from recurring units having the formula (B3), recurring units having the formula (B4), and recurring units having the formula (B5):

wherein R¹³ and R¹⁴ are each independently hydroxyl, halogen, acetoxy, an optionally halogenated C₁-C₈ alkyl group, optionally halogenated C₁-C₈ saturated hydrocarbyloxy group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyl group, or optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, with the proviso that R¹³ and R¹⁴ are not acid labile groups, R¹⁵ is halogen, acetoxy, an optionally halogenated C₁-C₈ alkyl group, optionally halogenated C₁-C₈ saturated hydrocarbyloxy group, optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyl group, or optionally halogenated C₂-C₈ saturated hydrocarbylcarbonyloxy group, with the proviso that R¹⁵ is not an acid labile group, c, d and e are each independently an integer of 0 to
 4. 11. The chemically amplified negative resist composition of claim 8, wherein the base polymer further comprises recurring units of at least one type selected from recurring units having the formula (B6), recurring units having the formula (B7), and recurring units having the formula (B8):

wherein R^(B) is each independently hydrogen or methyl, Z¹ is a single bond, or a C₁-C₆ aliphatic hydrocarbylene group, phenylene, naphthylene or a C₇-C₁₈ group obtained by combining the foregoing, or —O—Z¹¹—, —C(═O)—O—Z¹¹— or —C(O)—NH—Z¹¹—, Z¹¹ is a C₁-C₆ aliphatic hydrocarbylene group, phenylene, naphthylene or a C₇-C₁₈ group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety, Z² is a single bond or —Z²¹—C(═O)—O—, Z²¹ is a C₁-C₂₀ hydrocarbylene group which may contain a heteroatom, Z³ is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene, —O—Z³¹—, —C(═O)—O—Z³¹— or —C(═O)—NH—Z³¹—, Z³¹ is a C₁-C₁₀ aliphatic hydrocarbylene group, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene, or a C₇-C₂₀ group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety, R²¹ to R²⁸ are each independently halogen or a C₁-C₂₀ hydrocarbyl group which may contain a heteroatom, a pair of R²¹ and R²², R²³ and R²⁴, or R²⁶ and R²⁷ may bond together to form a ring with the sulfur atom to which they are attached, and M⁻ is a non-nucleophilic counter ion.
 12. The chemically amplified negative resist composition of claim 11, wherein the base polymer comprises recurring units having the formula (B1) and recurring units having the formula (B2), but not recurring units having the formula (B6), recurring units having the formula (B7) and recurring units having the formula (B8).
 13. The chemically amplified negative resist composition of claim 8, wherein the base polymer comprises recurring units having the formula (B1-2), recurring units having the formula (B2-2), and recurring units having the formula (B7):

wherein R^(A), R^(B), Z², R²³ to R²⁵, R^(x), R^(y), W¹¹, a¹ and b¹ are as defined above.
 14. The chemically amplified negative resist composition of claim 7, further comprising an organic solvent.
 15. The chemically amplified negative resist composition of claim 7, further comprising a quencher.
 16. The chemically amplified negative resist composition of claim 7, further comprising a crosslinker.
 17. The chemically amplified negative resist composition of claim 7, which is free of a crosslinker.
 18. The chemically amplified negative resist composition of claim 7, further comprising a fluorinated polymer comprising recurring units having the formula (D1) and recurring units of at least one type selected from recurring units having the formulae (D2) to (D5):

wherein R^(C) is each independently hydrogen or methyl, R^(D) is each independently hydrogen, fluorine, methyl or trifluoromethyl, R³⁰¹ is hydrogen or a C₁-C₅ straight or branched hydrocarbyl group which may have a heteroatom-containing moiety intervening in a carbon-carbon bond, R³⁰² is a C₁-C₅ straight or branched hydrocarbyl group which may have a heteroatom-containing moiety intervening in a carbon-carbon bond, R³⁰³, R³⁰⁴, R³⁰⁶ and R³⁰⁷ are each independently hydrogen or a C₁-C₁₀ saturated hydrocarbyl group, R³⁰⁵, R³⁰⁸, R³⁰⁹ and R³¹⁰ are each independently hydrogen, a C₁-C₁₅ hydrocarbyl group, C₁-C₁₅ fluorinated hydrocarbyl group, or acid labile group, an ether bond or carbonyl moiety may intervene in a carbon-carbon bond in the hydrocarbyl or fluorinated hydrocarbyl group, g¹ is an integer of 1 to 3, g² is an integer meeting 0≤g²≤5+2g³−g¹, g³ is 0 or 1, h is an integer of 1 to 3, X¹ is a single bond, —C(═O)—O— or —C(═O)—NH—, and X² is a C₁-C₂₀ (h+1)-valent hydrocarbon group or C₁-C₂₀ (h+1)-valent fluorinated hydrocarbon group.
 19. A process for forming a resist pattern comprising the steps of applying the chemically amplified negative resist composition of claim 7 onto a substrate to form a resist film thereon, exposing patternwise the resist film to high-energy radiation, and developing the exposed resist film in an alkaline developer.
 20. The process of claim 19 wherein the high-energy radiation is EB or EUV of wavelength 3 to 15 nm. 